1. Field of the Invention
The present invention relates to a process for producing sodium percarbonate having excellent stability and storability; sodium percarbonate prepared by this process and a bleach detergent composition containing the sodium percarbonate. In particular, the present invention relates to a process for producing sodium percarbonate suitable for incorporation into a bleaching agent composition as a main component or a detergent composition, which comprises the step of treating the surfaces of sodium percarbonate particles with specific coating agents used in the form of an aqueous slurry.
2. Description of the Related Art
Sodium percarbonate is well known as a base component which is incorporated into an oxygenic bleaching agent composition or a sterilizer composition, or as an oxidizer per se. Sodium percarbonate, which is usually produced by reacting sodium carbonate with hydrogen peroxide, is represented by the formula: 2Na.sub.2 CO.sub.3.3H.sub.2 O.sub.2 and comprises a sodium carbonate/hydrogen peroxide adduct. Sodium percarbonate is widely used in household or business bleaching agent compositions, since it is usable for bleaching colored or patterned cloths and it does not impair or yellow the cloth, though its bleaching power is slightly less than that of chloric bleaching agents at ambient temperature.
Another reason why sodium percarbonate attracts attention is that the decomposition products of this compound are free from any environmental pollution and therefore there is no problem of environmental pollution due to, for example, waste water.
However, sodium percarbonate is unstable and has problems such that the activity thereof is reduced during storage. In addition, when a very small amount of metals such as iron or copper are present as impurities, they act as a catalyst to accelerate a reduction in the activity of the sodium percarbonate. Further, since a problem of environmental pollution is created in semi-closed water areas, detergent compositions containing little or no phosphorus and in which synthetic zeolites instead of phosphorous compounds are utilized as a detergent builder, are recently in wide use. However, since zeolites also act as a catalyst to accelerate a reduction in the activity of sodium percarbonate, sodium percarbonate is extremely unstable in detergent compositions containing zeolite. That is, sodium percarbonate is catalytically decomposed by zeolite to rapidly lose available oxygen.
Sodium perborate is also utilized as a base component of oxygenic bleaching agent compositions other than sodium percarbonate. Although sodium perborate can be incorporated into detergent compositions or the like to form a relatively stable composition, it must be used at high temperatures, since its dissolution velocity in water is low. Thus sodium perborate is seldom used as a base component of an oxygenic bleaching agent composition in countries wherein low-temperature water is used for the washing of clothes and other works with water.
When sodium percarbonate having a high solubility at low temperature can be incorporated into a detergent composition to obtain a stable composition, bleaching and washing can be advantageously conducted at the same time, so that the development of a technique for improving the storability of sodium percarbonate is eagerly sought.
Various investigations have been made and various processes have been proposed for the stabilization of sodium percarbonate. For example, British Patent No. 1,575,792 (published on Oct. 1, 1980) discloses the use of boric acids, e.g., orthoboric acid, metaboric acid and tetraboric acid as coating agents for peroxides; and U.S. Pat. No. 4,526,698 (patented on Jul. 2, 1985; assignee: Kao Corp.) discloses the use of an aqueous borate solution to coat sodium percarbonate, and the coating of sodium percarbonate with an aqueous composite coating agent solution containing a borate and a silicate. Further, this U.S. Patent also discloses a process which comprises using an aqueous composite coating agent solution containing the above-described coating agents and sodium carbonate or the like as a second or third component, which is added and dissolved therein, to coat sodium percarbonate and spraying sodium percarbonate with the aqueous composite coating agent solution.
Sodium percarbonate produced by the process disclosed in British Patent No. 1,575,792, among the techniques of stabilizing sodium percarbonate, is still insufficient in its storability, coating strength and solubility. Further, although sodium percarbonate produced by the process described in U.S. Pat. No. 4,526,698 exhibits a remarkably improved storability when it is incorporated into a detergent composition, its storability is not yet comparable to that of sodium perborate and thus is not always satisfactory.
Investigations have been made also on a bleach detergent composition containing coated sodium percarbonate. For example, a bleach detergent composition containing sodium percarbonate having the surface coated with a borate-containing coating agent composition is described in the above-described U.S. Pat. No. 4,526,698. Further, it is described therein that the method for coating the surface of sodium percarbonate to be incorporated into the bleach detergent composition is not limited, and in an Example given therein, sodium percarbonate is coated with an aqueous borate solution.